Remove virtual destructor for WebVTTParser

Remove destructor for WebVTTParser as it was empty and marked as virtual.
This avoids creating an unnecessary vtable as this class is never subclassed
not does it have any virtual function.

Review URL: https://codereview.chromium.org/40323003

git-svn-id: svn://svn.chromium.org/blink/trunk@160499 bbb929c8-8fbe-4397-9dbb-9b2b20218538